Method of forming electric welded steel tube

Abstract

In a method of forming an electric welded steel tube, wherein a hot-rolled sheet is formed into a cylindrical shape, with the central portion thereof being lowered as the forming progresses, and thereafter, subjected to reduction in the circumferential direction of the tube by means of tandem type fin-pass rolls to be finished into the tube, the proper forming condition ranges capable of eliminating occurrence of edge waves in the tube seam edge portion and/or of cambers in the longitudinal direction of the tube by three forming conditions factors including the downhill value D H (downhill coefficient η) of the hot-rolled sheet, the fin-pass total reduction R of the tandem type fin-pass rolls and the distribution of the fin-pass reduction (the distribution ratio δ of the first fin-pass reduction).

What is claimed is: 
     
       1. A method of forming an electric welded steel tube, comprising: selecting a desired tube diameter;   calculating, using the desired tube diameter, a distance D H  such that a ratio η between the distance D H  and the desired tube diameter is within a range of values between 0.3 and 1.3;   feeding a heated sheet into a cage roller assembly;   downhill forming the heated sheet such that the calculated distance D H  is created between a level of feeding the sheet into the cage roller assembly and a lowered central portion of said heated sheet, thereby forming a cylindrical shape;   fin-pass forming the cylindrically shaped sheet and reducing a diameter of said cylindrical sheet by using tandem type fin-pass rollers, such that the total reduction R of said fin-pass forming is within a range of values between 0.4% and 1.5% and satisfying the following relationships:   R≧-1.44η+1.552       R≦0.51η+0.966       R≦-3.20η+4.86       R≧0.60η+0.08        wherein the fin-pass total reduction R is the sum of the reductions r i  of all of the fin-pass rollers of the cage roller assembly, the reduction r i  for each stand being   100 ln(l.sub.i -l/l.sub.i)        where l i  is the outer circumferential length of the tube at the outlet of the ith fin-pass roller;   selecting a distribution ratio δ of a first circumferential reduction of said fin-pass forming at a value of more than 50% of said total reduction R and satisfying the following relationships:   δ≧-200η+160       δ≧-75η-110       δ≧62.5η       δ≧250η-225        wherein the distribution ratio δ of the first circumferential reduction is   100 r.sub.1 /R        with r 1  being the reduction of the first fin-pass roller of the cage assembly;   heating longitudinal edges of said heat cylindrically shaped sheet;   welding the longitudinal edges of said heated cylindrically shaped sheet together to form a welded tube, whereby edge waves are prevented in the welded tube.   
     
     
       2. The method of claim 1, wherein the longitudinal edges of said heated cylindrically shaped sheet are heated after the fin-pass forming. 
     
     
       3. The method of claim 2, wherein the heating of the longitudinal edges occurs by induction. 
     
     
       4. The method of claim 3, wherein the longitudinal edges of the cylindrically shaped sheet are welded by upset butt welding. 
     
     
       5. A method of forming an electric welded steel tube, comprising: selecting a desired tube diameter;   calculating, using the desired tube diameter, a distance D H  such that a ratio η between the distance D H  and the desired tube diameter is within a range of values between 0.3 and 1.3;   feeding a heated sheet into a cage roller assembly;   downhill forming the heated sheet such that the calculated distance D H  is created between a level of feeding the sheet into the cage roller assembly and a lowered central portion of said heated sheet, thereby forming a cylindrical shape;   fin-pass forming the cylindrically shaped sheet and reducing a diameter of said cylindrical sheet by using tandem type fin-pass rollers, such that the total reduction R of said fin-pass forming is within a range of values between 0.4% and 1.5% and satisfying the following relationships:   R≧-0.542η+1.104       R≦1.750η+0.875       R≦-0.444η+1.533       R≦-6.00η+8.20        wherein the fin-pass total reduction R is the sum of the reduction r i  of all of the fin-pass rollers of the cage roller assembly, the reduction r i  for each stand being   100 ln(l.sub.i -l/l.sub.i)        where l i  is the outer circumferential length of the tube at the outlet of the ith fin-pass roller;   selecting a distribution ratio δ of a first circumferential reduction of said fin-pass forming at a value of more than 50% of said total reduction R and satisfying the following relationships:   δ≧-250η+125       δ≧-75η       δ≧250η-225        wherein the distribution ratio δ of the first circumferential reduction is   100 r.sub.1 /R        with r 1  being the reduction of the first fin-pass roller of the cage assembly;   heating longitudinal edges of said heated cylindrically shaped sheet;   welding the longitudinal edges of said heated cylindrically shaped together to form a welded tube, whereby a camber in the longitudinal direction of the tube is prevented.   
     
     
       6. The method of claim 5, wherein the longitudinal edges of said heated cylindrically shaped sheet are heated after the fin-pass forming. 
     
     
       7. The method of claim 3, wherein the heating of the longitudinal edges occur by induction. 
     
     
       8. The method of claim 4, wherein the longitudinal edges of the cylindrically shaped sheet are welded by upset butt welding. 
     
     
       9. A method of forming an electric welded steel tube, comprising: selecting a desired tube diameter;   calculating, using the desired tube diameter, a distance D H  such that a ratio between the distance D H  and the desired tube diameter is within a range of values between 0.3 and 1.25;   feeding a heated sheet into a cage roller assembly;   downhill forming the heated sheet such that the calculated distance D H  is created between a level of feeding the sheet into the cage roller assembly and a lowered central portion of said heated sheet, thereby forming a cylindrical shape;   fin-pass forming the cylindrically shaped sheet and reducing a diameter of said cylindrical sheet by using tandem type fin-pass rollers, such that the total reduction R of said fin-pass forming is within a range of values between 0.55% and 1.25% and satisfying the following relationships:   R≧-1.45η+1.555       R≦0.43η+0.991       R≦-0.42η+1.502       R≦-6.6η+8.920       R≧0.6η-0.080       R≧-0.51η+1.086        wherein the fin-pass total reduction R is the sum of the reduction r i  of all of the fin-pass rollers of the cage roller assembly, the reduction r i  for each stand being   100 ln(l.sub.i -l/l.sub.i)        where l i  is the outer circumferential length of the tube at the outlet of the ith fin-pass roller;   selecting a distribution ratio δ of a first circumferential reduction of said fin-pass forming at a value of more than 75% of said total reduction R and satisfying the following relationships:   δ≧-166.67η+150       δ≧75η       δ≧500η-525        wherein the distribution ratio δ of the first circumferential reduction is   100 r.sub.1 /R        with r 1  being the reduction of the first fin-pass roller of the cage assembly;   heating longitudinal edges of said heated cylindrically shaped sheet;   welding the longitudinal edges of said heated cylindrically shaped sheet together to form a welded tube, whereby a simultaneous prevention of edge waves in the tube edge portion and a camber in the longitudinal direction of the tube is achieved.   
     
     
       10. The method of claim 9, wherein the longitudinal edges of said heated cylindrically shaped sheet are heated after the fin-pass forming. 
     
     
       11. The method of claim 10, wherein the heating of the longitudinal edges occurs by induction. 
     
     
       12. The method of claim 11, wherein the longitudinal edges of the cylindrically shaped sheet are welded by upset butt welding.